The concept of a large area flat-screen television display which could be hung on a wall like a picture has been contemplated since the commercial introduction of the cathode ray tube television display in the 1940's. Despite pronouncements throughout the 1950's and 1960's that flat-screen television display would shortly become a commercial reality, that did not come to pass. The technical problems encountered in the development caused the cathode ray tube to remain essentially the exclusive way of reproducing video images.
The development of the liquid crystal active matrix flat-screen television display at Westinghouse Electric Corporation during the 1970's brought new life and substance to flat-screen television. See U.S. Pat. No. 3,840,695. This flat-screen display used nematic liquid crystal as an electro-optic medium which would transmit light, or not transmit light, depending on the electric potential applied across it. This display consisted of a glass substrate on which an orthogonal array (or "matrix") of thin film transistors, corresponding to picture elements (or "pixels") in the display, was deposited together with transparent contact pads spaced in a regular pattern, e.g., 1 millimeter on center, for contacting one surface of the liquid crystal layer. Conductive strips, in rows and columns, were also deposited between the picture elements over the substrate, the row strips being connected to the gate electrodes of the transistors and the column strips connected to the source electrodes of the transistors. The drain electrodes of the transistors are connected to the contact pads. A second common transparent contact was placed over the opposite side of the liquid crystal layer to allow a potential to be applied across it completing the picture elements of the matrix.
Each picture element of the display could be individually programmed, for each frame of a television picture, to the appropriate brightness, by storing a line of a frame in a peripheral horizontal analog shift register disposed at the top of the columns of the display. An entire row of switching transistors could then be activated by means of a vertically disposed digital shift register controlling the rows, which then resulted in the transfer of the analog voltage levels stored in the analog shift register at the top of the columns into the storage capacitors of that row. By repeating this sequence for each row of the matrix sequentially, in synchronism with the incoming video signal, the entire frame was constructed. In actual practice, the storage capacitor for each picture element could consist of the liquid crystal layer itself, thus simplifying the driving circuit to a single switching transistor at each picture element. With an analog shift register, an entire television frame could be generated by the timing and control circuit in real time, e.g., 33 msec. Also, the normally sluggish liquid crystal medium was able to show moving grey scale images of considerable perfection using this configuration. A color display was also produced by placing a patterned red-green-blue filter adjacent the active matrix so that each picture element could also be coordinated with the color components of a color video signal.
These active matrix liquid crystal flat-screen displays have been and are being made commercially for pocket size televisions. However, their size has been limited by the acceptable yield achievable with present manufacturing techniques. Typical active matrix liquid crystal displays have been 2 to 3 inches diagonal, although in development laboratories they have been made up to 10 inches diagonal. The latter, however, have not been made to my knowledge with acceptable yield. Moreover, even when such displays were successfully built, an added problem was the prospect of localized defects occurring in the display which could not be remedied without rejection or replacement of the entire display. Also, such liquid crystal displays needed a retaining wall to confine the liquid crystal in the image forming central area, and outside of this wall, substantial terminals were needed for each column and row of the matrix for interconnection with column and row driver circuits. For this reason, modular constructions of such displays were impractical, since the image area of each module was bounded by a wide opaque margin. It has been proposed to modularize the construction by limiting the column and row terminals to two or three sides, see U.S. Pat. No. 4,156,833; however, this limited the number of modules for a display typically to two or at most four, and in turn limited the size of the display.
Modular constructions of very large cathodo-luminescent and liquid crystal displays have been made for stadiums and the like. These displays, up to 25.times.40 meters in size, have been made with a large number of modules; however, they are characterized by very coarse resolution resulting from very large picture elements, e.g., one inch square. The boundaries of the modules in these large panels, particularly those constructed of liquid crystal modules, are visible in the resulting overall image, and produce a very undesirable effect.
Electroluminescent phosphors have also been contemplated for use in flat-screen television displays. This alternative, however, involves more sophisticated electronics. Electroluminescent phosphor displays operate at much higher voltages than liquid crystal displays, and in such an active matrix, two transistors and a capacitor are needed at each picture element to perform the switching function performed by one transistor in a liquid crystal display. See U.S. Pat. Nos. 4,006,383, 4,042,854 and 4,135,959.